Suitable crosslinked acrylic polymer particles of the type which may employ the stabilizer of this invention are well known. U.S. Pat. No. 4,147,688 to Makhlouf et al teaches crosslinked dispersions wherein crosslinked acrylic polymer microgel particles are formed by free radical addition polymerization of alpha, beta ethylenically unsaturated monocarboxylic acids, at least one other copolymerizable monoethylenically unsaturated monomer and a certain percentage of crosslinking monomer, in the presence of a hydrocarbon dispersing liquid (See abstract, examples and claims). Other crosslinked dispersions containing microgel particles are disclosed in the patent application and patents referred to in the Makhlouf et al disclosure.
U.S. Pat. No. 4,025,474 to Porter et al discloses a polyester based coating composition which includes the crosslinked dispersions disclosed by Makhlouf et al. U.S. Pat. No. 4,075,141 to Porter et al discloses carboxylic acid amide interpolymer-based coating compositions including the same crosslinked dispersions. U.S. Pat. No. 4,115,472 also to Porter et al, discloses urethane coating compositions also including the crosslinked dispersions of Makhlouf et al. U.S. Pat. No. 4,055,607 to Sullivan et al discloses thermosetting compositions of (a) solution acrylic polymer, (b) at least 0.5% of microgel particles formed by polymerizing hydroxyl bearing monomers with monhydroxyl bearing monomers in the presence of the stabilizer disclosed by Makhlouf et al, and (c) melamine resin. The microgel dispersion of Sullivan et al thus contains functionality capable of reacting with the melamine crosslinking agent.
The dispersion stabilizer employed in producing the microgel particles of the Makhlouf et al compositions are generally polymeric and contain at least two segments, with one segment being solvated by the dispersion liquid and the second segment being of different polarity than the first segment, and relatively insoluble, compared to the first segment, in the dispersing medium. Included among the dispersion stabilizers referred to in the Makhlouf et al patent are polyacrylates and methacrylates, such as poly(lauryl)methacrylate and poly(2-ethylhexylacrylate); diene polymers and copolymers such as polybutadiene and degraded rubbers; aminoplast resins, particularly high naphtha-tolerant compounds such as melamine formaldehyde resins etherified with higher alcohols (e.g., alcohols having 4 to 12 carbon atoms); and various copolymers designed to have desired characteristics (see Col. 5, lines 1-27).
Among the numerous dispersion stabilizers which could be employed in compositions of Makhlouf et al are those taught by U.S. Pat. No. 3,607,821 to Clarke. Clarke teaches a stabilizer for nonaqueous dispersions wherein the stabilizer is chemically reacted with dispersed particles of the dispersion (Col. 1, lines 36-42). Each co-reactant stabilizer molecule forms from 1 to 10 (preferably 1 to 4) covalent links with the dispersed polymer (Col. 1, lines 50-52). The covalent links between the stabilizer and the dispersed polymer are formed by reaction between chemical groups provided by the stabilizer and complementary chemical groups provided by the dispersed polymer or by copolymerization reaction (Col. 1, lines 63-67).
Particularly preferred dispersion stabilizers of Makhlouf et al and the general type of stabilizer employed in the preparation of particles of this invention are those which are graft copolymers comprising two polymeric segments with one segment being solvated by the dispersion liquid and not associated with polymerized particles of the polymerizable ethylenically unsaturated monomers and the second segment being an anchor polymer of different polarity from the first type and relatively non-solvatable by the hydrocarbon solvent and capable of anchoring with the polymerized particles of the ethylenically unsaturated monomer. This anchor polymer segment contains pendant groups capable of copolymerizing with the ethylenically unsaturated monomer used to form the particles of the dispersion (See Col. 5, lines 28-40 of Makhlouf et al).